Vietnamese environmental authorities said they were working with local officials in the Ha Tinh province to investigate dry waste, belonging to a unit of Taiwanese conglomerate Formosa Plastics Group, that was found dumped on a residential farm.

The probe puts Formosa Ha Tinh Steel under the spotlight yet again - this time for waste dumped inland in the province where earlier this year the firm released toxic waste into the sea and caused massive fish deaths in one of the biggest environmental disasters to ever hit Vietnam.

The latest investigation was ordered by the country's deputy prime minister, according to a statement published on the government's website earlier this week, while the environment minister has said that Formosa will be fined if the firm's dumping is proved to violate laws.

A representative for the steel plant, however, said the dry waste had been tested by the Vietnamese government last year and found to be non-toxic, adding that it had been improperly dumped by the contractor.

"We are checking with the Vietnamese government about a legal place to dump. We believe the testing they are doing this time will come back non-toxic as the first time," he added, declining to be named due to the sensitivity of the issue.

The government is retesting more than 100 metric tons of the dry waste, he said, adding the company expects to properly dispose of the waste this month or next after getting clearance from local authorities. The retest is an exercise in gaining public trust by the government, he added.

The toxic disaster caused by the Formosa unit earlier this year had led to a huge outcry, with months of public anger on social media and on the streets of big cities. Vietnamese vented their fury at the government and Formosa, one of the communist country's biggest investors, accusing them of a cover-up.

Formosa later pledged $500 million in damages and admitted its $10.6 billion steel plant had caused massive fish deaths along a 200-km (124-mile) stretch of coastline in April.

Vietnamese state media also reported that people in Thien Cam, about 60 km away from the Formosa steel plant to the north, suspect waste in a landfill there came from Formosa as well. Ha Tinh's environment department has taken waste samples to test, according to local media reports.